Developer electrode for electrophotographic apparatus



Sept. 13, 1960 DEVELOPER ELECTRODE FOR Filed Feb. 5, 1955 H. E. CLARK ETAL ELECTROPHOTOGRAPHIC APPARATUS 2 Sheets-Sheet 1 JNVENTOR. HAROLD E. CLARK ROBERT w GLJNDLACH ATTORNEY Sept. 13, 1960 H. E. CLARK EI'AL DEVELOPER ELECTRODE FOR ELECTROPHOTOGRAPHIC APPARATUS Filed Feb. 3, 1955 2 Sheets-Sheet 2 FIG.4

INVENTOR. HAROLD E. CLARK ROBE/37' w GLJHDLACH ATTORNEY United States iPatent .ce

DEVELOPER ELECTRODE FOR ELECTROPHOTO- GRAPHIC APPARATUS Harold E. Clark, Rochester, and Robert W. Gundlach, Spencerport, N.Y., assignors to Haloid Xerox Inc., Rochester, N.Y., a corporation of New York Filed Feb. 3, 1955, Ser. No. 485,866 9 Claims. .(Cl. 118-637) The present invention relates to electrophotographic techniques, and more particularly to an improved developer electrode for use in apparatus adapted to develop a powder image of the insulating image layer of a xerographic plate.

In the production of Xerographic copies, the photoconductive insulating image layer of the xerographic plate is first charged electrostatically, the charged surface then being exposed under light to the subject to be copied, thereby forming an electrostatic image. Thereafter the image layer with the electrostatic image thereon is subjected to a cascading operation of electroscopic or developer powder to produce a powder image, which is then transferred and ainxed to paper or other transfer medium.

In the making of powder images on insulating layers that have previously been electrostatically charged, it has been found that undesirable effects arise in the background areas due to the adherence of portions of the electroscopic powder to the non-image areas of the plate on which the image layer is supported. The powder that adheres to the non-image areas is carried onto the transfer sheet along with the powder on the image areas. Such powder as remains on the non-image areas is subsequently charged and causes a build-up of powder and a continual increasing of undesirable effects in the background areas in printing operations.

The present invention has particular reference to techniques involving the use of developing materials consisting of a combination of finely divided pigmented electroscopic powder with a coarser carrier material. The carrier material is triboelectrically charged with a polarity opposite to that of the electroscopic powder upon frictional engagement therewith and acts to retain the electroscopic powder which is attracted to and surrounds the particles of carrier material. If the charge on the non-image areas of the plate has a greater attractive force for the electroscopic powder particles than has the charge on the carrier material, the electroscopic powder particles are likely to be attracted to and held by the nonimage areas on the plate, even though such non-image areas are of a conductive character, to drain off the major part of the electrostatic charge except over the insulating image areas.

It has hitherto been suggested to impose a further electrostatic charge on the non-image areas of the plate or to surround said areas with an electrostatic field such that the charge on the non-image areas will have a polarity opposite to the charge on the image areas and the same as the charge on the electroscopic powder and will therefore repel such powder and prevent its adherence to the non-image areas. For this purpose a charged developer electrode has been incorporated in continuous printing machines or in other mechanisms for applying electroscopic powder layer, whereby the powder will be applied evenly and to a charged image Patented Sept. 13, 1960 uniformly over the image areas of the plate and the nonimage areas will be left substantially free of electroscopic powder.

Thus, in the Patent 2,573,881, issued to Walkup et al., a rigid developer electrode is disclosed operating in conjunction with a rotary mechanism. Also in the co-' pending patent application of Gundlach, Serial No. 408,713, filed February 8, 1954, there is disclosed a rigid developer electrode operating in conjunction with a developer apparatus.

In the device disclosed in said co-pending patent application, the developer electrode is in close proximity to the Xerographic plate, and the electric potential applied thereto is relatively low.

Experience has now shown that the use of a rigid developer electrode with a cascading developer apparatus gives rise to certain practical difiiculties which adversely affect the development process. With a rigid electrode it is essential that the spacing from the Xerographic plate be parallel and uniform throughout. In practice, this is diflicult to attain. Moreover, with a rigid electrode the developing powder tends to agglomerate in certain areas. The developer carrier and toner particles at times will pile up between the plate and developer electrode. This pile-up in efiect creates a block preventing the cascading developer from passing beyond the block, thereby preventing particle deposition or substantially diminishing particle deposition in the areas following the block. Another serious aspect of the tendency of a developer to pile up in areas is the likelihood of damage to the surface of xerographic plates. inasmuch as the sensitive surface is rather brittle, movement of the surface while a pile-up exists is likely to result in permanent damage thereof.

' pile-up of powder.

Still another object of the invention is to provide a flat flexible developer electrode structure for a tray developing apparatus. 1

Yet another object of the invention is to provide a curved flexible developer electrode structure for a rotary development apparatus.

For a better understanding of the invention as well as other and further features the following detailed description to be read in conjunction with the accompanying drawing, wherein like components in the several views are identified by like reference numerals.

In the drawings:

Fig. l is a longitudinal sectional view taken centrally through the powder receptacle, showing a developer electrode structure in accordance with the invention in conjunction with a Xerographic plate.

Fig. 2 is a perspective view solely of the developer electrode structure.

Fig. 3 is a schematic diagram of the circuit connecting the voltage source of the developer electrode with the metal backing of the xerographic plate.

Fig. 4 is a diagrammatic sectional View illustrating the powder development section of a continuous electrode.

printing apparatus.

thereof, reference is made to Fig. is a perspective view solely of the developer electrode employed in conjunction with the apparatus shown in Fig. 4.

Fig. 6 is a sectional view of the developer electrode structure shown in Fig. 5.

Referring now to the drawings, and more particularly to Fig. 1, there is illustrated a powder receptacle of the general character illustrated in Patent 2,600,580 dated. June 17, 1952, and comprising a top wall 10 and a bottom Wall 11. Bottom wall 11 has an opening therein defined by lateral flanges 12 which extend about the opening and support compressible strips 13 formed of rubber or the like. A xerographic plate, generally designated by numeral 14, is held against strips 13 by pressure ap plied thereto by spring catches 15 when the plate is positioned in the powder receptacle. 7

The powder receptacle with the xerographic plate held therein is oscillated to cascade an electroscopic or developer powder 50 alternately in opposite directions over the plate in a manner similar to that disclosed in said Patent 2,600,580. Developer powder 50 preferably takes the form disclosed in Patent 2,618,551 dated November 18, 1952, or that in Patent 2,618,552 bear-ing the same date. In general, such powders comprise a granular carrier material consisting of a core encased in a suitable covering which imparts triboelectric properties to the granular carrier material so that it will properly charge the electroscop-ic powder when mixed therewith, while maintaining such a relative specific gravity as to ensure against adherence of a granular carrier material to the charged plate.

The present invention is concerned primarily with an improved developer electrode structure, generally designated by numeral 16, which is yieldably supported within the powder receptacle. The developer electrode structure in accordance with the invention is constituted by a frame 17 and a flexible electrode plate 18 mounted thereon. Thus, when cascading the developer powder over the image surface of the xerographic plate, the powder travels in the space between the image surface and the developer electrode plate 18 which is arranged in close proximity to the xerographic plate during the powder developing operation.

As best seen in Fig. 2, the developer electrode structure is formed by a rigid metallic frame 17 having a generally rectangular shape, the opposing ends 17a and 17b of the frame being extended and curved upwardly to define deflectors for directing the powder into the space between the xerographic place and the developer plate and to prevent the main body of the developer powder 50 from falling into the central area of the powder receptacle above the developer electrode during the cascading operation. Secured to the under surface of frame 17 is'the flexible electrode plate 18 which is constituted by a rectangular sheet which is insulating on one side and electrically conductive on the other. This sheet may be made from materials such as cloth, paper, webs of all types, rubber and the like, having a conductive coating applied on one side and an insulating coating on the other.

If an insulating material is used, such as different plastic materials, it is only necessary to coat a conductive layer on one side; whereas if a conductive material is employed, then an insulating layer must be applied to one side thereof. The following combinations represent preferred forms of flexible electrode plates: a satin material with a coating of Aquadag, which is a colloidal solution of'graphite in water; a thin rubber material with the same conductive coating thereon; a vinyl plastic material similarly coated with conductive material. The insulating material should be relatively thin, since this makes possible closer spacing of the conductive surface of the flexible electrode to the xerographic plate. Suitable for this purpose is vinyl plastic sheeting having a thickness of .003 inch.

The flexible electrode plate 18 is secured to frame 17 so that the insulation side thereof faces the image-bearing surface of the xerographic plate 14. This allows physical contact between the image-bearing layer and the flexible developer electrode to take place Without causing distortion through discharge of the charge pattern on the image-bearing surface. It will be appreciated that by using a flexible developer electrode plate having some freedom of movement throughout areas of its surface, contact with the xerographic plate may result.

The flexible electrode plate 18 is held tautly on the frame, this being accomplished, for example, by taping ends of the plate 18 to the corresponding ends 17a and 17b of the frame. Thus, the flexible plate 18 is free to bulge away from the xerographic plate when necessary to pass developer powder through the space between the developer electrode and the xerographic plate in cascading operation. The rectangular window of frame 17 will, of course, permit swelling of the flexible plate 18.

To suspend the developer electrode structure above the xerographic plate 14 and to maintain a desired spacing between the flexible electrode plate 18 and the plate 14, the frame 17 is provided with spacer elements 19 which are attached to the frame adjacent each corner thereof and are dimensioned to positon the flexible developer electrode as close as possible to the xerographic plate without interfering with the flow of powder. In addition, two pairs of suitable springs 20 are provided attached to wall 10 of the receptacle, each pair terminating in a metal pusher strip 21 which bears against the ends of frame 17 and yieldably supports the developer electrode structure in operative position relative to the xerogra-phic plate. While springs 20 have been shown as helical in form, it is to be understood that flat springs or other resilient means may be employed to support the electrode structure.

The electrical circuit of the apparatus is shown in Fig. 3. In this figure the developer electrode structure 16 is shown schematically in section as constituted by metal frame 17, conductive surface 18a of the flexible electrode plate and insulating layer 18b of said plate. The xerogr'aphic plate 14 is shown as constituted by a photo-conductive surface 14b and a conductive backing 14a therefor. The electrical circuit includes a suitable voltage source 51 connected at one end through a resistor 52 to metal frame 17, which is in electrical contact with the conductive layer 18a of the flexible electrode. The other end of voltage source 51 is connected to the conductive backing 14a of the xerographic plate. Voltage source 51 may be formed by three or four 30-volt batteries connected in series and housed within the powder receptacle, or the batteries may be disposed in an external housing.

To develop an electrostatic charge pattern on the imagebearing surface, it is desirable that electric lines of force be drawn from the image-bearing surface with equal effect. In the case of a rigid developer electrode, as disclosed in the above-identified co-pending application, this is accomplished by maintaining an exact and proper spacing between the electrode and the xerographic plate. However, when using a flexible electrode, in accordance with the invention, the spacing may not be instantaneously uniform throughout. It is only necessary to attain equal average spacing of the flexible electrode from all areas of the image-bearing surface. By obtaining such equal average spacing the benefits of rigid spacing are realized without severe mechanical tolerances.

Development of images depends on field strength and Field strength is a function of area.

development time. charge or charge density and electrode spacing when the electrode is assumed to be at ground potential. When the electrostatic image pattern exists on the image-bearing surface, areas of the surface possess varying charge densities which depend on and are related to the light pattern being reproduced. In order to develop areas in propor tion to their charge densities, it is necessary to create an external field strength pattern corresponding to the charge pattern on the plate surface. This field pattern is created properly through the use of a flexible developer electrode which is spaced at an equal average distance across the image-bearing surface. This spacing results in equal representation of field strength over areas of the imagebearing surface having equal charges and proportionately different representations over proportionately different charged areas. Since development time is the same over all areas on the image-bearing surface and inasmuch as field strength extending outwardly from this surface is proportional to the charge pattern, then the image reproduced is a true representation of the existing charge pattern.

The distance the developer electrode is spaced from the xerographic plate surface is controlled by two basic factors: (a) its influence on electrostatic field strength, and (b) that distance which will allow the cascading powder particles to move over the image-bearing surface with suflicient freedom to bring about proper development. The closer the electrode is to the image-bearing surface the greater is its influence on the electrostatic field strength. When using a flexible electrode which can dis tend to allow particles to move between the electrode and the image-bearing surface, proper spacing is attained by suspending the electrode above the surface of the image- 'bearing layer and by maintaining it taut, in the manner disclosed in connection with Fig. l. The electrode may be positioned touching the plate surface if the flow of developer powder is not interfered with detrimentally. This is dependent on such factors as the particular developer particles being used, the force with which they are introduced between the electrode and the image-bearing layer, the flexibility of the electrode, and the like.

The spacing between the flexible electrode and the xerographic layer may be adjusted within broad limits, as from just out of contact to a distance of one-half inch, the preferred spacing being between just out of contact to of an inch. A variable factor also to be considered is the thickness of the photo-conductive layer on the xerographic plate. sing thick selenium layers it is possible to use greater spacing of the developer electrode than when using relatively thin selenium layers. Also to be considered is the strength of the charge pattern itself. When using charges of approximately 600 volts it is possible to use greater spacings of the developer electrode than when using charges of 50 volts.

A further advantage of the flexible electrode structure shown in Fig. 1 resides in the fact that with cascading operation the travel of particles between the electrode and the image-bearing surface causes an air movement which results in a reduction in pressure or a negative pressure in the restricted zone between the xerographic plate and the developer electrode. The negative pressure between the developer electrode and the image-bearing layer causing a sucking-in of the flexible electrode toward the image-bearing layer, thereby maintaining as close as possible the spacing throughout the development cycle.

The xerographic plate, as pointed out above, when charged usually has a potential of approximately 600 volts, while after exposure a charge of 10 to 30 volts remains on the background or discharged areas, with substantially the full charge remaining on the undischarged or image areas. The developer electrode plate 18 causes the lines of force from the undischarged areas to be directed in generally parallel relationship toward electrode plate 18 and in perpendicular relationship to the image surface. This results in uniform adherence of the developer powder over the image area and produces a unifonnly dark or concentrated image with lines that are not diffused, as contrasted with powder developing procedures in which there is no developer electrode in parallel relation to the xerographic plate, and in which the lines of force from the edges or outer portions of an image area are consequently directed in curved paths to the adjacent discharged or background areas, resulting in the lines of force being dissipated from the image area and producing black or dark effect around the edges and light or white effect at the central portions.

The potential imposed on the flexible developer electrode 18 is preferably from 60 to volts with a polarity the same as that of the charged image areas and the same as the polarity of the charge on the background area of the xerographic plate but with a greater voltage than that of the charge on the background areas, resulting in an electrical field which neutralizes the field caused by the residual charge on the discharged or background areas. In this manner, the flexible electrode 18 prevents powder from adhering to the discharged or background areas, and produces a copy having uniformity in both the image and background areas, with better contrast there between and greatly improved quality of copy.

The invention is also applicable to a continuous electro-printing machine, which is partially shown in Fig. 4 and comprises a rotating drum 22 having mounted thereon a conductive xerographic printing plate 23 carrying the insulating image layer (not shown). Also provided is a receptacle 24 from which electroscopic powder 25 gravitates over the inclined bottom wall 26 to fall onto the drum and travel downwardly by gravity, the surplus powder which is not retained on the charged image layer falling into a receptacle 27. The continuous paper strip, the corona discharge needles and other conventional element of the continuous machines are not shown herein, such element being similar to that disclosed, for example, in the Patent 2,573,881 issued November 6, 1951.

In order to prevent adherence of electroscopic powder to the non-image areas of the printing plate there is provided an arcuate developer electrode structure, generally designated by numeral 28, preferably curved as shown to conform to the curvature of the drum and located between the powder chamber wall 29 and spaced from the drtuns. Apart from the curvature, the electrode structure 28 is substantially similar to that shown in Fig. 1, and comprises a rectangular metal frame 30 and a flexible electrode plate 31 constituted, for example, by an insulating sheet 31a having a conductive layer 31b thereon, the insulating side facing the printing plate 23. The sheet 31 is secured at its sides to the sides of the frame, whereby the sheet conforms in shape to that of the frame. The developer electrode extends from a point immediately beneath the inclined bottom 26 to a point below that at which the powder falls away from the drum, so that-the electrode substantially covers the entire area over which powder is in moving contact with the drum.

The electrode structure is yieldably mounted on chamber wall 29 by means of suitable springs 32, the electrode structure being spaced from the drum and riding thereon by means of rollers 33 mounted adjacent the corners of the frame. Alternatively, the arcuate electrode structure may be rigidly mounted from the chamber wall in spaced relation to the drum.

The use of a rotary drum in automatic machines requires a greater amount of cascading particles at greater flow rates than in the case of flat plates which are manually developed. With such greater flow rates and greater amounts of particles, a rigid developer electrode, such as shown in Patent 2,573,881, would necessarily be separated from the printing plate by relatively large distances, such as .5 inch or more, in that the position of the electrode would be determined by the requirements of particle flow. However, the relatively large separation of rigid developer electrode and printing plate substantially cuts down the effect of the electrode in drawing out the electrostatic field lines of force. On the other hand, the flexible electrode structure herein disclosed permits closer spacing to the rotary drum, thereby enhanc-' pattern with ing the electrostatic effects on the charge 7 out adversely affecting the movement of the cascading particles.

While there has been disclosed what are at present considered to be preferred embodiments of the invention, it is to be understood that many changes and modifications may be made therein without departing from the essential aspects of the invention. It is intended therefore in the appended claims to cover all such modifications as fall within the true scope of the invention.

What is claimed is:

1. In an apparatus for developing an electrostatic latent image on the insulating layer of a xerographic element, the combination of an electrode supporting frame, means to maintain said frame in spaced relation to a xerographic element supported in said apparatus, and a flexible electrode including a conductive layer suspended in said frame wherein said electrode supporting frame is positioned to maintain the electrode spaced from a xerographic element within the range from just out of contact to a distance of one-half inch and the electrode and a xerographic element so supported define a path for developing material, whereby the spacing of the electrode to a xerographic element is varied in accordance with the pressure applied against the electrode by developing material passing therebetween.

2. In an apparatus for developing an electrostatic latent image on the insulating layer of a xerographic plate, the combination of an electrode supporting frame, means to maintain said frame in spaced relation to a xerographic plate supported in said apparatus, and a flexible electrode suspended in said frame, said electrode including a conductive layer and an insulating layer, said insulating layer facing a xerographic plate so supported wherein said electrode supporting frame is positioned to maintain the electrode spaced from a xerographic plate within the range from just out of contact to a distance of one-half inch and the electrode and a xerographic plate so sup ported define a path for developing material, whereby the spacing of the electrode to a xerographic plate is varied in accordance with the pressure applied against the electrode by developing material passing therebetween.

3. In an apparatus for developing an electrostatic latent image on the insulating layer of a xerographic plate, the combination of an electrode supporting frame, means to maintain said frame in spaced relation to a xerographic plate supported in said apparatus, and a flexible electrode suspended in said frame, said electrode including a conductive layer and an insulating layer, said insulating layer facing a xerographic plate so supported, said electrode and a plate so supported defining a path for developing powder, said conductive layer having an electrical charge thereon to establish an electrostatic field between said electrode and a plate so supported wherein said electrode supporting frame is positioned to maintain the electrode spaced from a xerographic plate within the range from just out of contact to a distance of onehalf inch and the electrode, whereby the spacing of the electrode to a xerographic plate is varied in accordance with the pressure applied against the electrode by developing material passing therebetween.

4 In an apparatus for developing an electrostatic latent image on the insulating layer of a xerographic plate, the combination of an electrode supporting frame, means to maintain said frame in spaced relation to a xerographic plate su ported in said apparatus, and a flexible electrode sus ended in said frame, said electrode including a conductive layer and an insulating layer, said insulating layer facing a xerographic plate so supported, said insulating layer being constituted by a vinyl plastic sheet, said conductive layer being constituted by an aquadag coating on said plasticsheet wherein 'said electrode supporting frame is positioned to maintain the electrode spaced from a xerographic plate within the range from just out of contact to a distance of one half inch and the electrode and a xerographic plate so supported define a path for developing material, whereby the spacing of the electrode to a xerographic plate is varied in accordance with the pressure applied against the electrode by developing material passing therebetween.

5. In an apparatus for developing an electrostatic latent image on the insulating layer of a xerographic plate, the combination of an electrode supporting frame, means to maintain said frame in spaced relation to a xerographic plate supported in said apparatus, and a flexible electrode suspended in said frame, said electrode including a conductive layer and an insulating layer, said insulating layer facing a plate so supported, said insulating layer being constituted by a satin sheet, said conductive layer being constituted by an aquadag coating on said sheet wherein said electrode supporting frame is positioned to maintain the electrode spaced from a xerographic plate within the range from just out of contact to a distance of onehalf inch and the electrode and a xerographic plate so supported define a path for developing material, whereby the spacing of the electrode to a xerographic plate is varied in accordance with the pressure applied against the electrode by developing material passing therebetween.

6. In an apparatus for developing an electrostatic latent image on the insulating layer of a xerographic plate, the combination of an electrode supporting frame, means to maintain said frame in spaced relation to a xerographic plate supported in said apparatus, and a flexible electrode suspended in said frame, said electrode including a conductive layer and an insulating layer, said insulating layer facing a plate so supported, said insulating layer being constituted by a rubber sheet, said conductive layer being constituted by a graphite colloid wherein said electrode supporting frame is positioned to maintain the electrode spaced from a xerographic plate within the range from just out of contact to a distance of one-half inch and the electrode and a xerographic plate so supported define a path for developing material, whereby the spacing of the electrode to a xerographic plate is varied in accordance with the pressure applied against the electrode by developing material passing therebetween.

7. Apparatus for developing an electrostatic latent image on the insulating layer of a flat xerographic plate comprising a powder receptacle, means for supporting a xerographic plate on said receptacle, an electrode supporting frame, means to maintain said frame in spaced relation to a xerographic plate supported on said receptacle, a flexible electrode including a conductive layer suspended in said frame, and means to impose an electrical charge on said conductive layer wherein said electrode supporting frame is positioned to maintain the electrode spaced from a xerographic plate within the range from just out of contact to a distance of one-half inch and the electrode and a xerographic plate so supported define a path for developing material, whereby the spacing of the electrode to a xerographic plate is varied in accordance with the pressure applied against the electrode by developing material passing therebetween.

8. Apparatus for developing an electrostatic latent image on the insulating layer of a flat xerographic plate comprising a powder receptacle, means for supporting a xerographic plate on said receptacle, and a developer electrode structure arranged in said receptacle, said electrode structure including a rigid frame, resilient means to urge the frame against a plate supported on said receptacle, spacer elements fixed on the frame to maintain the frame in spaced relation to a plate so supported, and a flexible electrode supported on said frame wherein said frame is positioned to maintain the electrode spaced from a xerographic plate within the range from just out of contact to a distance of one-half inch and the electrode and a xerographic plate so supported define a path for developing material, whereby the spacing of the electrode to a xerographic plate is varied in accordance with the pressure applied against the electrode by developing material passing therebetween.

9. Apparatus for developing an electrostatic latent image on the insulating layer of a fiat Xerographic plate comprising a powder receptacle, means for supporting a Xerographic plate on said receptacle, and a developer electrode structure arranged in said receptacle, said electrode structure including a rigid rectangular frame, resilient means to urge the frame against a plate supported on the receptacle, and a flexible electrode supported on said frame and provided with a conductive layer and an insulating layer, said insulating layer facing a plate so supported, said frame being provided with spacers to maintain a predetermined spacing between said electrode and a xerographic plate supported on the receptacle wherein said frame is positioned to maintain the electrode spaced from a Xerographic plate within the range from just out of contact to a distance of one-half inch and the electrode and a xerographic plate so supported define a path for developing material, whereby the spacing of the electrode to a Xerographic plate is varied in accordance with the pressure applied against the electrode by developing material passing therebetween.

UNITED STATES PATENTS Wintermute Sept. 12, 1939 Meston Nov. 19, 1940 Melton et al. Aug. 26, 1941 Carlson Mar. 17, 1942 Carlson Sept. 12, 1944 Walkup et al. Nov. 6, 1951 Carlson Mar. 11, 1952 Sahel et al. June 17, 1952 Lang Dec. 29, 1953 Walkup et al. Sept. 14, 1954 Schwartz et al. Aug. 16, 1955 Landrigan et al. Nov. 29, 1955 Landry Apr. 17, 1956 Jacob July 3, 1956 Gundlach Jan. 15, 1957 Lowrie Aug. 20, 1957 OTHER REFERENCES Oughton: Decoration of Glass and Ceramic Articles by Xerography, The Glass Industry, vol. 30, No. 12, December 1949, pages 662, 653 and 66-4, 

